1. Field of the Invention
This invention relates to integrated circuits, and more particularly, to differential amplifiers implemented as integrated circuits.
2. Description of the Related Art
Differential amplifiers are frequently employed to indicate when an input signal is greater than or equal to a certain value. The amplifier can be configured to change output states from one logic level to another when a voltage signal is applied between the positive and negative inputs of the amplifier. In this configuration the amplifier and associated circuitry are typically referred to as a voltage or level detector.
The voltage level at which the detector will trip (change output states) is typically established by applying a constant reference voltage to the negative input of the amplifier and a variable signal to the positive input. The output of the detector will be at one state until the voltage at the positive input is greater than or equal to the reference voltage. At this point the positive differential voltage at the amplifier input will produce a shift in the amplifiers output state. For example, it may be desired to have an indication of when a signal is greater than or equal to half the supply voltage, Vcc, for a certain system. A voltage divider network in which both resistors have the same value may be used to bias the negative input of the differential amplifier to Vcc/2. Under these circumstances the output of the level detector should change states each time the input voltage passes through the Vcc/2 point. If the output is at logic 0 when the input is less than Vcc/2, then it should change to logic 1 when the input becomes greater than or equal to Vcc/2.
Two such level detector circuits may be combined to produce a window detector. Often it is desirable to produce an indication that an input signal is within a certain window, i.e. that the signal is greater than some minimum voltage level and less than some maximum level. Such a detection circuit may be realized by appropriately combining the outputs of two level detectors, which have been biased for trip points at the minimum and maximum boundary voltage levels. For example, in a personal computer system it may be desired to produce an indication when a power supply voltage exceeds or falls below its nominal value by a certain percentage in order to insure reliable functioning of the system.
Optimally, the amplifier/detector should change states for any positive differential voltage applied between the positive and negative inputs no matter how small. In real world implementations, however, certain physical parameters limit and/or move the trip point of the amplifier. For differential amplifiers implemented in CMOS technology, the trip point may be affected by the threshold voltage of the transistors used to construct the circuit. The transistor threshold voltage may be dependent on fabrication parameters such as doping levels, the dielectric constant of the gate insulating material, and feature geometry among others. While many of these fabrication parameters may be fairly tightly controlled across transistors within a single die, significant variances may occur from die to die or from one wafer to another. This means that the trip point of one detector IC may be somewhat different from the trip point of another IC built to the same design simply due to production process variations.
Another factor that may cause variance in the trip point of a differential amplifier is the temperature at which it operates. The threshold voltage of CMOS transistors is directly dependent on operating temperature and furthermore the dependence for PMOS devices is different than that for NMOS devices.
In typical implementations, the differences in trip point caused by production process variations as well as temperature over the useful range of the circuit may be on the order of a few millivolts. In many applications, the voltage being detected is on the order of several volts, so the trip point accuracy may be within a few tenths of a percent. However, some applications may require the detection of voltage levels on the order of several tens of millivolts. The same process and temperature variations can produce voltage detection errors on the order of 10% in these applications. Therefore, it may be desirable to implement a differential amplifier whose trip point has a high immunity to temperature and fabrication process variations.